Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A method for operating an apparatus which has an oscillatable unit. The
oscillatable unit is excited to oscillate by means of a first frequency
sweep within a predetermined frequency band with successive, discrete
exciter frequencies of increasing or decreasing frequency. A first
exciter frequency is ascertained, in the case of which, during the first
frequency sweep, at least one predeterminable criterion is fulfilled. The
oscillatable unit is excited by means of a second frequency sweep,
wherein the frequency band, compared with the first frequency sweep, is
run through in the opposite direction. A second exciter frequency is
ascertained, in the case of which, during the second frequency sweep, the
at least one predeterminable criterion is fulfilled. From the first
exciter frequency and the second exciter frequency, via formation of an
average, a measuring frequency for determining and/or monitoring at least
one process variable is determined.

Claims:

1-11. (canceled)

12. A method for operating an apparatus for determining and/or monitoring
at least one physical process variable of a medium, which apparatus has
an oscillatable unit, comprising the steps of: oscillating the
oscillatable unit to oscillate by means of a first frequency sweep within
a predetermined frequency band in the working range of the oscillatable
unit using transmission signals with successive, discrete exciter
frequencies of increasing or decreasing frequency; receiving
corresponding oscillations of the oscillatable unit in the form of
received signals; ascertaining a first exciter frequency, in the case of
which, during the first frequency sweep, at least one predeterminable
criterion is fulfilled; oscillating the oscillatable unit to oscillate by
means of a second frequency sweep within the predetermined frequency band
in the form of transmission signals with the same successive, discrete
exciter frequencies; running through the frequency band, compared with
the first frequency sweep, in opposite direction; receiving the
corresponding oscillations of the oscillatable unit in form of said
received signals; ascertaining a second exciter frequency, in the case of
which, during the second frequency sweep, the at least one
predeterminable criterion is fulfilled; and determining and/or monitoring
from the first exciter frequency and the second exciter frequency, via
formation of an average, measuring frequency at least one process
variable.

13. The method as claimed in claim 12, wherein: ascertained as the
average measuring frequency is the resonance frequency or a frequency, in
the case of which a predeterminable phase shift between transmission
signal and received signal is present, especially an eigenfrequency of
the oscillatable unit.

14. The method as claimed in claim 12, wherein: predetermined as
criterion for determining the first exciter frequency and the second
exciter frequency is the presence of a maximum or minimum amplitude
and/or a particular phase shift between the transmission signal and the
received signal.

15. The method as claimed in claim 12, wherein: a difference between the
first exciter frequency and the second exciter frequency is ascertained
and from the difference between the first exciter frequency and the
second exciter frequency, the degree of damping of the oscillations is
determined.

16. The method as claimed in claim 15, wherein: from the degree of
damping of the oscillations, viscosity of the medium is determined.

17. The method as claimed in claim 15, wherein: based on the degree of
damping in the case of oscillation in air, it is ascertained whether
accretion is clinging to the oscillatable unit or whether the
oscillatable unit is corroded.

18. The method as claimed in claim 16, wherein: progression of the degree
of damping is monitored as a function of time, and therefrom, a change in
viscosity or accretion or corrosion is detected.

19. The method as claimed in claim 12, wherein: after determining the
average measuring frequency, the oscillatable unit is excited to
oscillate with the measuring frequency for determining and/or monitoring
at least one process variable.

20. The method as claimed in claim 12, wherein: after determining the
average measuring frequency, the average measuring frequency is evaluated
as regards at least one process variable, and another set of first and
second frequency sweeps for another ascertaining of the average measuring
frequency are performed.

21. The method as claimed in claim 12, wherein: as the process variable,
at least one of the variables from the group comprising fill level,
density, viscosity, mass flow and volume flow is determined and/or
monitored.

22. An apparatus for performing a method comprising the steps of:
oscillating the oscillatable unit to oscillate by means of a first
frequency sweep within a predetermined frequency band in the working
range of the oscillatable unit using transmission signals with
successive, discrete exciter frequencies of increasing or decreasing
frequency; receiving corresponding oscillations of the oscillatable unit
in the form of received signals; ascertaining a first exciter frequency,
in the case of which, during the first frequency sweep, at least one
predeterminable criterion is fulfilled; oscillating the oscillatable unit
to oscillate by means of a second frequency sweep within the
predetermined frequency band in the form of transmission signals with the
same successive, discrete exciter frequencies; running through the
frequency band, compared with the first frequency sweep, in opposite
direction; receiving the corresponding oscillations of the oscillatable
unit in form of said received signals; ascertaining a second exciter
frequency, in the case of which, during the second frequency sweep, the
at least one predeterminable criterion is fulfilled; and determining
and/or monitoring from the first exciter frequency and the second exciter
frequency, via formation of an average, measuring frequency at least one
process variable; the apparatus comprising: a mechanically oscillatable
unit; a drive/receiving unit, which excites said oscillatable unit by
means of transmission signals to execute mechanical oscillations and
receives oscillations of said oscillatable unit and converts these into
electrical, received signals; and an electronics unit, which at least
evaluates the received signals.

Description:

[0001] The present invention relates to a method for operating an
apparatus for determining and/or monitoring with an oscillatable unit at
least one physical process variable of a medium, as well as to an
apparatus for performing the method. The oscillatable unit is, for
example, an oscillatory fork, a single rod, or a membrane introducible
into a medium. With an apparatus having such an oscillatable unit, fill
level, density, and/or viscosity of a medium can be determined. The
oscillatable unit can likewise be embodied as an oscillatable tube, which
is flowed through by the measured medium and by means of which process
variables such as flow, density, or viscosity are measurable. An
apparatus, which includes one or more of such tubes, is, for example, a
Coriolis flow measuring device.

[0002] Known for determining process variables of a medium in a container
are, among others, vibronic measuring devices. These are offered by the
assignee in a large number of variants under the marks Liquiphant for
liquids and Soliphant for bulk goods. Such vibronic measuring devices
have an oscillatable unit in the form of an oscillatory fork, which is
composed of a membrane and two paddles protruding therefrom into the
medium, or in the form of an oscillatable rod. The oscillatable unit is
excited to resonant oscillations, and a change in the oscillation
frequency and/or the amplitude of the oscillations and/or the phase
between the transmission signal and the received signal is evaluated with
respect to the process variable, principally a limit-level or the density
of the measured medium. In the case of a vibronic limit level switch for
liquids, a distinction is made, for example, between the free state, i.e.
a freely oscillating oscillatory fork, and the covered state, i.e. an
evaluated oscillatory fork covered by medium. The two states have
different resonance frequencies. Furthermore, with such an apparatus,
also density and viscosity of the medium are determinable or a phase
boundary is detectable.

[0003] Other vibronic measuring devices are known for the field of flow
measurement. In such case, an oscillatable tube is inserted, as an
intermediate piece, into a pipeline flowed through by medium and is
excited to oscillations. Via the Coriolis effect, flow velocity is
determinable. From the oscillatory behavior, however, information
concerning density and viscosity of the medium can also be obtained.

[0004] Excitation for execution of mechanical oscillations most often
occurs by means of a piezoelectric drive, in the case of which at least
one piezoelectric element coupled with the oscillatable unit is supplied
with an electrical transmission signal, which it converts to a mechanical
signal. Conversely, the mechanical oscillations of the oscillatable unit
can be converted into evaluatable electrical signals by means of a
piezoelectric receiving unit. Often, the drive unit and receiving unit
are embodied as a combined drive/receiving unit and, together with a
control/evaluation unit, are arranged in a control loop. This control
loop controls the transmission signal in such a manner that a
predeterminable phase is present between the transmission signal and
received signal.

[0005] In Offenlegungsschrift DE 102009026685 A1, an alternative method
for digitally controlled excitation of the oscillatable unit to
mechanical oscillations is disclosed. In contrast to the analog
embodiment, in the case of this largely digital solution, a forced
excitation occurs at a particular frequency. In order to find that
measuring frequency, in the case of which the predetermined phase shift
exists between the received signal and transmission signal, a so-called
frequency sweep is performed. In a frequency sweep, the oscillatable unit
is excited to oscillations within a particular frequency band in the
working range of the sensor and successively with discrete frequencies
lying close to one another, and the frequency corresponding to the
predetermined phase shift is ascertained. DE 102009028022 A1 describes an
advantageous further development of the frequency sweep, which simplifies
evaluation of the received signal for finding the measuring frequency, in
that the received signal phase is selectively sampled and evaluated only
at certain points in time.

[0006] A problem arises in the case of said method when only a small
amount of time is available for the frequency sweep. The faster the
frequency band is run through at a constant number of excited
frequencies, the smaller the time, which is available to the oscillatory
system to adapt itself to the newly presented frequency. Superpositioning
effects occur, which lead to the predetermined phase shift occurring at a
frequency different from the actually sought measuring frequency, and
thus to the measuring frequency not being correctly ascertainable.

[0007] An object of the invention is to provide a method, which enables
finding a measuring frequency rapidly and with high accuracy.

[0008] The object is achieved by a method for operating an apparatus for
determining and/or monitoring at least one physical process variable of a
medium. The apparatus has an oscillatable unit. The oscillatable unit is
excited to oscillate by means of a first frequency sweep within a
predetermined frequency band in the working range of the oscillatable
unit using transmission signals with successive, discrete exciter
frequencies of increasing or declining frequency. The corresponding
oscillations of the oscillatable unit are received in the form of
received signals. A first exciter frequency is ascertained, in the case
of which, during the first frequency sweep, at least one predeterminable
criterion is fulfilled. The oscillatable unit is excited to oscillate by
means of a second frequency sweep within the predetermined frequency band
in the form of transmission signals with the same successive discrete
exciter frequencies, wherein this frequency band, compared with the first
frequency sweep, is run through in the opposite direction. The
corresponding oscillations of the oscillatable unit are received in the
form of received signals; and a second exciter frequency is ascertained,
in the case of which, during the second frequency sweep, the at least one
predeterminable criterion is fulfilled. From the first exciter frequency
and the second exciter frequency, via formation of an average, a
measuring frequency for determining and/or monitoring at least one
process variable is determined.

[0009] The terminology "measuring frequency" means that frequency, which
is of relevance for determining and/or monitoring at least one process
variable. For determining the process variable, either the value of the
measuring frequency itself, or the signal received from exciting the
oscillatable unit with the measuring frequency is evaluated. Furthermore,
both the measuring frequency as well as also the received signal recorded
at the measuring frequency can be evaluated.

[0010] According to the invention, two frequency sweeps are performed one
after the other, and the results are combined with one another. For this
purpose, the frequency band is run through once with frequencies of
increasing level and once with frequencies of decreasing level. In such
case, it is not important whether the frequency band is run through first
in a decreasing or increasing direction. The parameters of the two
frequency sweeps should, in such case, be essentially identical, i.e. the
same frequencies are excited, the sampling rate is constant and the
duration of the frequency sweeps is equal, so that the same period of
time lies between the exciting of two successive frequencies. The two
frequency sweeps thus differ only in the direction, in which the
frequency sequence is progressed through. Sampling and evaluation of the
individual frequency sweeps can occur in manner known from the state of
the art.

[0011] The measuring frequency determined as the average value of the two
ascertained exciter frequencies is independent of amplitude errors or
phase errors, which occur due to beat effects in the received signal of a
sweep. The errors in the case of determining the exciter frequency in an
upwards sweep and a downwards sweep are compensated during the average
formation, so that the measuring frequency is determinable reliably and
with high accuracy. The accuracy is, in such case, not dependent on the
time required for first or second frequency sweep, so that the
determining of the measuring frequency is ascertainable even in shortest
amount of time without losses in accuracy. Since the dynamic properties
of the sensor are compensated for, the achievable accuracy is only
determined and thus adjustable by the frequency step width.

[0012] A further advantage of the method lies in the fact that in the case
of continuous exciting with frequency sweeps, due to the exciting with
frequencies in reverse order following a particular frequency sweep, no
large jumps in the excitation frequency occur, and the demands made on
the electronic components, such as, for example, adaptive filters, can
thus be correspondingly smaller. The method thus not only saves time for
performing the measuring, but also saves costs in terms of installed
components.

[0013] The method is, in principle, applicable for all measuring devices
of vibration type. This includes both fill level measuring devices,
density measuring devices and viscosity measuring devices, as well as
also flow measuring devices.

[0014] In a first embodiment of the solution of the invention, ascertained
as the measuring frequency is the resonance frequency or a frequency, in
the case of which a predeterminable phase shift between the transmission
signal and received signal is present, especially the eigenfrequency of
the oscillatable unit. The eigenfrequency corresponds to the frequency,
in the case of which a phase shift of 90° is present.

[0015] In a further development of the method of the invention,
predetermined as the criterion for determining the first exciter
frequency and the second exciter frequency is the presence a maximum or
minimum amplitude and/or a particular phase shift between the
transmission signal and the received signal.

[0016] In an additional further development of the invention, the
difference between the first exciter frequency and the second exciter
frequency is ascertained, and from the difference between the first
exciter frequency and the second exciter frequency, the degree of damping
of the oscillations is determined.

[0017] In an embodiment of the invention, from the degree of damping of
the oscillations, the viscosity of the medium is determined. The
association occurs on the basis of a stored relationship between the
degree of damping and viscosity and/or between the frequency difference
and viscosity.

[0018] An embodiment of the method includes that based on the degree of
damping in the case of an oscillation in air, it is ascertained whether
accretion is clinging to the oscillatable unit or whether the
oscillatable unit is corroded. Whether the oscillatable unit oscillates
in air, can, for example, be detected based on the eigenfrequency or
resonance frequency.

[0019] An embodiment of the invention provides that progression of the
degree of damping is monitored as a function of time, and a change of the
viscosity or an accretion formation or corrosion formation is detected
therefrom.

[0020] In a further development of the method, after determining the
measuring frequency, the oscillatable unit is excited to oscillate with
the measuring frequency for determining and/or monitoring at least one
process variable. In this case, the frequency sweeps first and foremost
serve for determining the measuring frequency, with which the
oscillatable unit is excited at least during a certain time period
subsequent to the sweeps. Due to the excitation with only one particular
frequency, no superimposing of oscillations of different frequencies
occur, so that the signal received in such case is representative for at
least one process variable to be determined.

[0021] An alternative embodiment includes that, after determining the
measuring frequency, the measuring frequency is evaluated as regards at
least one process variable, and another set of first and second frequency
sweeps for another ascertaining of the measuring frequency is performed.
If the process variable can already be ascertained from the measuring
frequency, an interruption of the exciting by means of frequency sweeps
is not required. Due the continuous performing of the frequency sweeps
for determining the measuring frequency, the current measuring frequency
is always known.

[0022] In an embodiment of the invention, as the process variable, at
least one of the variables from the group comprising fill level, density,
viscosity, mass flow and volume flow is determined and/or monitored.

[0023] Additionally, the object of the invention is achieved by the
features of an apparatus for performing the method of the invention. This
apparatus comprises: a mechanically oscillatable unit; a drive/receiving
unit, which excites the oscillatable unit by means of transmission
signals to mechanical oscillations and receives oscillations of the
oscillatable unit and converts these into electrical received signals;
and an electronics unit, which at least evaluates the received signals.
Such apparatuses for determining one or more process variables of a
medium are known by the name "vibratory measuring device". In the case of
a first group of measuring devices, the oscillatable unit is, at least at
times, immersed in the medium, and is embodied, for example, as an
oscillatory fork, a membrane, a rod, or a closed pipe.

[0024] In the case of a second group, the oscillatable unit is, for
example, one or more tubes, through which the medium flows at least at
times.

[0025] The invention will now be explained in greater detail based on the
appended drawing, the figures of which show as follows:

[0031] FIG. 1 shows a so called oscillatory fork as an example of a
vibronic fill level measuring device 1, with which also density and/or
viscosity of the medium 3 are measurable. The oscillatable unit 2 is
formed by a membrane and two paddles, wherein the paddles are arranged
symmetrically on the membrane. The membrane closes a tubular housing 11
at the latter's end, wherein, arranged in such housing 11 are, for
example, the drive/receiving unit 6 for producing and receiving the
mechanical oscillations of oscillatable unit 2, as well as an electronics
unit 5 for open and/or closed loop control of the oscillation excitement
and for evaluating the received signals and determining the process
variables. Preferably, drive/receiving unit 6 is an electromechanical
transducer, especially a piezoelectric stack drive, which is in contact
with the inner side of the membrane and thus excites the oscillatable
unit 2 to oscillate.

[0032] The fill level measuring device 1 is installed in the wall of the
container 4 at a fill level to be monitored. In this embodiment, a
maximum fill level is monitored; the monitoring of a minimum fill level,
for example, as running dry protection for downstream pumps, is, however,
likewise possible. Before the fill level of the medium 3 reaches the
maximum level, the oscillatable unit 2 oscillates in air; upon reaching
and exceeding this maximum fill level, the oscillatable unit 2 becomes
covered by medium 3. This results in a change in the resonance frequency
and also in the eigenfrequency. Such a change is detected for fill level
monitoring. The illustrated apparatus 1 can likewise by applied as a
density and/or viscosity measuring device, since density and viscosity
likewise have an influence on the oscillation characteristic. The degree
of covering of oscillatable unit 2 with the process medium 3 should,
however, be known for determining these two process variables.

[0033] With the method of the invention, the measuring frequency can be
rapidly and precisely ascertained. For this, two frequency sweeps with
the same parameters are performed, in that a predetermined frequency band
is run through with a plurality of discrete, successive frequencies for
exciting the mechanically oscillatable unit. The frequency of the
transmission signal is thus continually changed. The two frequency sweeps
differ only in the direction of movement through the predetermined
frequency band.

[0034] The goal of the two frequency sweeps is, in each case, to ascertain
a particular exciter frequency f1, f2, which fulfils at least
one predeterminable criterion. The criterion is selected in such a manner
that two corresponding frequencies are ascertained. "Corresponding"
refers to the received signals of the frequency sweeps being symmetrical,
and consequently, points corresponding to one another in amplitude and
relative position in the respective received signal can be determined.
These corresponding frequencies have in terms of magnitude an equal
difference with respect to the frequency, which the symmetry axis
determines and which corresponds to the sought measuring frequency. Via
average formation, the measuring frequency fm is determined from the
two exciter frequencies f1, f2.

[0035] In an embodiment, the eigenfrequency of the system is ascertained.
For this, based on the two sweeps, a frequency f1, f2 is in
each case determined, in the case of which, during the sweep, the
transmission signal and received signals have a particular phase shift
with respect to one another. The average value of these two exciter
frequencies f1, f2 then gives the actual measuring frequency
fm, in the case of which the predetermined phase shift is present
independently of a sweep. The eigenfrequency determined thusly can be
established as the exciter frequency, so that the oscillatable unit is
subsequently excited at least for a particular period of time with the
eigenfrequency. This enables, for example, the determining and evaluation
of the associated amplitude.

[0036] As a rule, the phase shift is predetermined at 90°, in order
to excite the oscillatable unit with the eigenfrequency and to determine
the fill level or density. A high viscosity can, however, influence the
measuring. So that the viscosity does not act as a disturbing variable, a
phase shift different from 90° can be specified, which, for
example, lies between 40° and 70°.

[0037] In an alternative embodiment, the current resonance frequency is
determined by means of the method of the invention. Following this, the
method can be repeated, so that the current resonance frequency is
continually determined. This can be evaluated as regards a process
variable to be determined or monitored, for example, the fill level of
the medium 3.

[0038] FIG. 2 shows a measuring transducer of Coriolis type, by means of
which flow, density and/or viscosity of a medium are determinable. Such a
measuring device 1 is pressure-tightly inserted in a pipeline as an
intermediate piece, for example, by means of flanges. The oscillatable
unit 2 is a measuring tube, through which, at least at times, medium is
flowing. Equally, the oscillatable unit 2 can comprise a number of
measuring tubes. For measuring, the measuring tube is excited with the
resonance frequency to oscillate. The controlling of the measuring
frequency usually occurs with a PLL (phase locked loop). With help the
method of the invention, the resonance frequency of the system can be
found, so that this frequency is settable as the starting frequency for
PLL. In this way, it is prevented that the PLL locks on a disturbance
frequency instead of the resonance frequency.

[0039]FIG. 3 shows an amplitude frequency diagram with recorded and
calculated signals. In this diagram, the received signal R.sup.+ of a
first frequency sweep and the received signal R.sup.- of a second
frequency sweep are presented. The mechanical oscillations of the
oscillatable unit 2 leading to these received signals R.sup.+, R.sup.are essentially undamped. In the case of the first frequency sweep, the
frequency band was progressed through with increasing frequency, while
the frequencies in the second frequency sweep were excited in a
decreasing direction. Further presented is the magnitude of the ideal
curve of the received signal IR, which is present when no modulations due
to mutually superimposing oscillations occur. The peak lies at the
resonance frequency.

[0040] Moreover, for two frequency sweeps, a signal CR.sup.+, CR.sup.calculated from the respective received signal R.sup.+, R.sup.- is
presented, whose maxima are associated with frequencies, in the case of
which the received signal R.sup.+, R.sup.- and the transmission signal
have a predetermined phase shift relative to one another.

[0041] The smaller the sweep time--that is the duration, which is
available for a frequency sweep in the case of a predetermined separation
of frequencies to be excited for movement through the predetermined
frequency band--the more strongly the received signals R.sup.+, R.sup.deviate from the ideal signal IR. This is to be attributed to beat
phenomena in the received signal R.sup.+, R.sup.-, which lead to a
corruption of the amplitude and phase information. The maximum amplitude
of the received signal R.sup.+, R.sup.- consequently does not occur at
the actual resonance frequency. Likewise, a point with a particular phase
shift between the transmission signal and received signal shifts, as is
to be seen in the marked main maxima of the calculated signals CR.sup.+,
CR.sup.-.

[0042] The illustrated received signals R.sup.+, RR.sup.- of the first
frequency sweep and of the second frequency sweep are mirror symmetrical
about a line, which extends parallel to the ordinate and which intersects
the abscissa at the resonance frequency. The correct resonance frequency
can, consequently, be determined by forming the average value of the two
frequencies, at which the maxima occur in the received signals R.sup.+,
R.sup.- of the increasing and decreasing frequency sweeps.

[0043] In order to ascertain two corresponding exciter frequencies
f1, f2 of the first frequency sweep and the second frequency
sweep, at least one criterion is established, which the received signal
R.sup.+, R.sup.- or a signal CR.sup.+, CR.sup.- derived from the received
signal R.sup.+, R.sup.- must fulfill, either alone or in connection with
the transmission signal or a signal derived therefrom. Two corresponding
exciter frequencies f1, f2 have an equal difference with
respect to the sought measuring frequency fm, wherein the difference
is distinguished in the sign. The average value of the first exciter
frequency f1 and the second exciter frequency f2 corresponds,
consequently, to the sought measuring frequency fm.

[0044] If the resonance frequency is the sought measuring frequency
fm, preferably that frequency is in each case ascertained as first
exciter frequency f1 and, respectively, second exciter frequency
f2, in the case of which the respective received signal has a global
maximum. An alternative criterion for determining the resonance frequency
is the presence of a particular local maximum or minimum. By a
"particular local maximum or minimum" is meant the i-th, for example, the
first, second or third, of the N local maxima or minima occurring in the
recorded received signal.

[0045] For determining a measuring frequency, in the case of which a
predeterminable phase shift between the transmission signal and received
signal is present, a requirement for phase shift between transmission
signal and received signal present during the particular frequency sweep
is established as a criterion. As a rule, a number of frequencies exist,
in the case of which a particular phase shift occurs, so that the
fulfillment of an additional criterion is required for a unique
determining of the exciter frequencies f1, f2. Preferably, from
all frequencies, in the case of which a predetermined phase shift is
present, that frequency f1, respectively f2 is determined, in
the case of which the corresponding amplitude of the received signal is
highest. This case is presented in FIG. 3.

[0046] For case in which the phase shift is set at 90° and the
damping is so small that the elgenfrequency and resonance frequency of
the oscillatable unit are the same, the measurement frequencies fm
determined by means of said different criteria coincide, and correspond
to the frequency, at which the ideal received signal IR has the maximum.

[0047] Besides the criteria named by way of example, also other criteria
for finding the two exciter frequencies f1, f2 arranged
symmetrically around the measuring frequency fm to be ascertained
are, of course, useable. The choice of the best suited criterion or best
suited combination of a plurality of criteria ultimately depends on the
measuring frequency fm to be ascertained--for example, resonance
frequency or frequency in the case of which a particular phase
relationship exists, especially eigenfrequency--as well as on the
embodiment of the electronics unit and the evaluating algorithms
available.

[0048] A concrete embodiment of the method of the invention will now be
explained in the following in greater detail on the basis of the flow
diagram illustrated in FIG. 4. As measuring frequency fm, that
frequency should be determined, in the case of which between the
transmission signal and received signal, a predeterminable phase shift of
90° exists.

[0049] In a first step A, the parameters of the two desired frequency
sweeps, as well as the property of the measuring frequency fm to be
determined are established. In this case, the property of measuring
frequency fm lies in the fact that, at the measuring frequency
fm, a phase shift of 90° between transmission signal and
received signal is present. The algorithm for evaluation of the received
signals is correspondingly fixed, or criteria are correspondingly
established. The frequency band, in which the sought measuring frequency
fm lies, is, for example, predetermined by the geometry of the
oscillatable unit and is, consequently, known. Furthermore, the step
width, i.e. the distance between two excited frequencies, and the sweep
time, i.e. the time, which is used for a frequency sweep, are fixed. Via
the step width, the resolution of the recorded received signal is
predeterminable.

[0050] In a second step B, the exciting of the oscillatable unit 2 occurs
with a first frequency sweep in an increasing direction, i.e. with the
lowest frequency of the frequency band at the beginning. Likewise, one
could begin with a decreasing sweep. At the same time, the particular
received signal R.sup.+ is sampled and processed. The evaluation
advantageously occurs with the algorithm described in DE 102009028022 A1.
By means of the phase selective sampling described there, a calculated
received signal is produced, from whose maxima, all frequencies are
determinable, in the case of which, for measured received signal, the
predetermined phase shift exists between transmission signal and received
signal, in this example 90°. A received signal calculated in such
a manner is presented in FIG. 3. From all frequencies, which fulfill the
phase condition, that exciter frequency--called the first exciter
frequency f1--is ascertained, in the case of which the received
signal or calculated received signal possesses the greatest amplitude.

[0051] In a next step C, a second frequency sweep is performed with the
same parameters as the first frequency sweep, but in a decreasing
direction. In other words, frequency band, step width and sweep time are
equal. Solely the direction, in which the frequency band is run through,
is different from the first frequency sweep. The received signal Rrecorded during the second frequency sweep is evaluated in a manner equal
to the earlier recorded received signal R.sup.+, and the second exciter
frequency f2 is ascertained analogously to first exciter frequency
f1.

[0052] In the next step D, from the first exciter frequency f1 and
the second exciter frequency f2, the sought measuring frequency
fm is determined by forming the average value of the two ascertained
exciter frequencies, i.e. (f1+f2)/2.

[0053] Optionally, in a next step E, the difference f1-f2
between first exciter frequency f1 and second exciter frequency
f2 is determined, and therewith, the degree of damping D of the
medium 3 is ascertained. From this, the viscosity of the medium 3 can be
ascertained, to the extent that the oscillatable unit 2 is covered during
the sweep with medium 3, or information can be extracted concerning the
state of oscillatable unit 2 with respect to accretion or corrosion, to
the extent that oscillatable unit 2 oscillates uncovered.

[0054] In a first variant, the determining of the measuring frequency
fm is followed in a step F by the exciting of the oscillatable unit
2 at least for a certain time period with the measuring frequency
fm. From the received signal gained in the case of monofrequent
excitation, at least one process variable is then determined or
monitored, for example, based on the amplitude and/or measuring frequency
fm. For example, the fill level can be determined or monitored based
on measuring frequency fm and the associated amplitude. The method
for measurement frequency determination can then, for example, be
performed again at regular intervals.

[0055] In a second variant, the determining of measuring frequency fm
is followed by, as next step G, evaluation of the measuring frequency
fm as regards a process variable, for example, the density or fill
level. This is followed by another run-through, i.e. another performing
of an increasing and a decreasing frequency sweep, the ascertaining of
the two exciter frequencies f1, f2 and the determining of the
measuring frequency fm. In this case, sweeps are thus virtually
continually performed.

[0056]FIG. 5 shows the dependence of the magnitude of the difference
|f1-f2| between the first exciter frequency f1 and the
second exciter frequency f2 on the degree of damping D, also called
"Lehr's measure of damping". The smaller the damping of the oscillations,
the stronger are the arising modulations and the greater the distance
between the first exciter frequency f1 and the second exciter
frequency f2.

[0057] From the difference |f1-f2| of the two exciter
frequencies, which were ascertained by means of two frequency sweeps
performed in opposing directions, the degree of damping D is thus
measurable. This is a measure for viscosity of the medium. Preferably, in
the electronics unit of the measuring device, a relationship between
frequency difference and viscosity is stored, for example, in the form a
characteristic curve, formula or table. During operation of the measuring
device, the frequency difference |f1-f2| between first exciter
frequency f1 and second exciter frequency f2 can then be
ascertained, and therefrom, the viscosity of the medium can be
determined. Such a viscosity determination is especially advantageous,
since the viscosity is virtually incidentally determinable, while by
means of the measuring frequency fm, another process variable is
ascertainable, especially independently of the viscosity.

[0058] The degree of damping D furthermore enables a diagnosis of the
state of the oscillatable unit. For this, progression of the degree of
damping D in time is monitored, for example, in the case of oscillation
in air, or a current degree of damping D is compared with a starting
value. The starting value can be ascertained, for example, at the
start-up of the apparatus. If accretion has formed on the oscillatable
unit, the damping of the oscillations is higher than without accretion.
Via the determining of the frequency difference, also a detection of
accretion is thus implementable. Since accretion also affects the
reliability of the determining of other process variables, detection of
accretion enables assuring reliable measurements. In the case of
accretion formation, a warning signal is preferably output to the
operating personnel or to a control room.

[0059] Conversely, from a decrease in the damping in the case of constant
environmental parameters, corrosion of the oscillatable unit can be
detected. Evaluation of the frequency difference |f1-f2| is
thus not only advantageous for viscosity determination, but also likewise
as regards predictive maintenance.